igneous
Granitic Pegmatite
Coarse-grained Igneous Rock (Granitic composition: KAlSi3O8, SiO2)
Hardness: 6-7 (Mohs scale); Color: Mottled white, cream, and black; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline (very large crystals); Cleavage: Perfect in feldspar, none in quartz; Specific gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled white, cream, and black
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled white, cream, and black; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline (very large crystals); Cleavage: Perfect in feldspar, none in quartz; Specific gravity: 2.6-2.7
Formation & geological history
Formed during the final stage of magma crystallization where high water content allows for rapid ion migration and massive crystal growth. Typically found in late-stage intrusive plutons or as dikes/veins.
Uses & applications
Used as a primary source for rare-earth elements, industrial minerals like mica and feldspar, and occasionally as structural stone or architectural slabs.
Geological facts
Pegmatites are famous for hosting the world's largest crystals; some spodumene crystals in pegmatites have reached lengths of 40 feet. They are also the primary source of many gemstones like tourmaline and beryl.
Field identification & locations
Identify in the field by looking for exceptionally large intergrown crystals of quartz and feldspar exceeding 1 cm. Common in mountain ranges and ancient shields. Collectors should look for 'pockets' within the rock which may contain high-quality crystals.
